In an attempt to clarify rotavirus immunology and as a means of investigating rotavirus protein structure and function we have isolated a series of monoclones directed at several rotavirus proteins. Monoclones were derived from mice immunized with the Wa strain of human rotavirus, the rhesus 2 strain of simian rotavirus and a human rotavirus (DS-1) reassortant (41-2). We studied 70 monoclones directed at 5 viral proteins. Monoclone production utilized standard techniques and assay systems including RIA, neutralization, immunoprecipitation and hemagglutination-inhibition (HI). The monoclones were characterized by the proteins to which they react: (1) Monoclones to the 42,000 dalton inner protein, the 6th gene product; two of these have subgroup specificity. (2) Monoclones to the 34,000 dalton surface glycoprotein, the 8th or 9th gene product; some of these neutralize the virus to high titer and some have HI activity. (3) Monoclones to the 82,000 dalton surface protein, the 4th gene product, the viral hemmaglutinin; these monoclones inhibit hemagglutination and neutralize to low titer. (4) Monoclones to the 28,000 dalton 10th gene product, the 3rd viral surface protein; one of these monoclones neutralizes infectivity and one has low level HI activity. (5). Two monoclones appear to react with either the 2nd or 3rd viral protein, an 88,000 dalton inner protein.